1. Field of the Invention
The invention relates to a method of producing a nitrided lithium-transition metal compound oxide useful as a positive electrode active material, for example, the nitrided lithium-transition metal compound oxide, and a lithium-ion battery.
2. Description of the Related Art
The lithium-ion battery has a high electromotive force and a high energy density, and has been widely put into practice in the field of information-related equipment and communication equipment. Meanwhile, in the field of automobiles, the development of electric vehicles and hybrid vehicles is an urgent business in terms of the environmental problem and the resource problem and as the power source for these vehicles, lithium-ion batteries are being studied. The lithium-ion battery typically has a positive electrode active material layer containing a positive electrode active material, a negative electrode active material layer containing a negative electrode active material, and an electrolyte layer formed between the positive electrode active material layer and the negative electrode active material layer.
It has been known that compounds that have a rock-salt layer structure, such as LiCoO2 and compounds that have a spinel structure, such as LiMn2O4, are used as a positive electrode active material for a lithium-ion battery. Meanwhile, a method of nitriding such a compound (lithium-transition metal compound oxide) with the use of ammonia is available, Japanese Patent Application Publication No. 2006-32321 (JP-A-2006-32321) describes a method of producing an active material, in which, after heating an oxide that has a resistivity of 1×104 Ωcm or more in a reducing atmosphere, the oxide is reacted with ammonia gas to obtain a nitrided oxide that is expressed by a composition formula of LixMeOyNz, where 0≦x≦2, 0.1<y<2.2, 0<z<1.4 and Me is at least one selected from the group consisting of Ti, Co, Ni, Mn, Si, Ge, and Sn. Meanwhile, although not a method of producing a positive electrode active material, a method of nitriding an oxide with the use of urea is available. Japanese Patent Application Publication No. 2002-154823 (JP-A-2002-154823) describes a method of producing an inorganic oxynitride having a photocatalytic activity by heating a mixture of an oxide (titanium oxide, for example) that has a particular specific surface area and a nitrogen compound (urea, for example) that is adsorbed by the oxide at room temperature.
Although nitriding a lithium-transition metal compound oxide with the use of ammonia is already known as described in JP-A-2006-32321, when the ammonia is used, nitriding reaction is hard to occur and therefore, the improvement of the electron conductivity is limited.